U.N. diplomats are accusing Sudan of using delay tactics to prevent the world body from taking action to prevent atrocities in the war-ravaged Darfur region.

U.N. peacekeeping officials say the process of transforming a beleaguered African Union mission in Darfur to a robust U.N. force is beset by a myriad of obstacles. The handover had been scheduled for late this year, but numerous delays are threatening an indefinite postponement.

The prospect of delays comes as U.N. emergency relief coordinator Jan Egeland is warning that a massive humanitarian aid operation in the region is on the verge of collapse. Three years of civil war in Darfur have killed an estimated 200,000 people and left two million more homeless.

Deputy Undersecretary for Peacekeeping Operations Hedi Annabi recently visited Sudan and Ethiopia to discuss mission handover with Sudanese and African Union leaders. He said the Khartoum government is refusing to permit what he called "essential" planning for the handover until Dafur's warring factions reach agreement in peace talks currently under way in Abuja, Nigeria.

"If you work backward, there is a sequence that is always there when we prepare a peacekeeping operation," said Hedi Annabi. "So as one is delayed, the other steps will be delayed, and this is one point I made in Khartoum that if we cannot move ahead with the technical assessment mission, that will delay our preparations and we may not be as ready as we would like if and when they call on us to help."

The United States, which has labeled the massacres of Darfur civilians as genocide, has contributed more than $1 billion in aid for the region, and offered logistics and planning expertise for the peacekeeping transition. A frustrated U.S. Ambassador to the U.N. John Bolton Wednesday called Sudan's refusal to allow a U.N. assessment mission to visit Darfur "inexcusable."

"The argument that it has to wait for Abuja or it has to wait for the consent of Khartoum, this is just delaying and delaying and delaying, and it's consistent with pattern that the Sudanese government has followed for years in this," he said. "The government of Sudan continues to resist, and if they drive process, we will not have a transition, and that's not something that we find acceptable."

Wednesday's meeting came a day after the Security Council voted to impose sanctions against four Sudanese accused of war crimes in Darfur. British, French and American diplomats called the sanctions a way to exert added pressure for agreement at the Abuja peace talks.

Britain and the United States suggested they might push for harsher penalties, and add new names to the list, if atrocities in Darfur continue.